1. Field of the Invention
The present invention relates to an optical unit for use in a laser beam printer, and more particularly to an optical unit which guides a laser beam from a laser diode to an object, by way of a group of focusing lenses and a scanner.
2. Description of the Related Art
In general, an optical unit incorporated in a laser beam printer or the like is designed such that a laser beam output from a laser diode is guided first to a scanner and then to a photosensitive body, i.e., an object to be scanned. The photosensitive body is scanned with the laser beam at a constant speed. When guided from the laser diode to the photosensitive body, the laser beam passes through a group of focusing lenses. By these focusing lenses, the laser beam is made to have a cross sectional shape having desirable characteristic, and then fall on the predetermined position on the surface of the photosensitive body.
This type of optical unit is made up of first and second optical systems which are isolated from each other. The first optical system converges the laser beam output from the laser diode, while the second optical system focuses the laser beam on the photosensitive body. The scanner is arranged between the first and second optical systems, as will be detailed later.
The first optical system is a combination of lenses, such as an aspheric glass lens, plastic lenses, etc. The laser beam generated from the laser diode is first converted into a substantially collimated laser beam by a collimator lens included in the first optical system, and is then guided to the second optical system. The second optical system is a combination of lenses, such as an f.theta. lens. By this f.theta. lens, the deflection angle at which a laser beam is deflected by the scanner is changed in proportion to the position at which the laser beam is focused on the photosensitive body and which is expressed in relation to the main scanning direction.
U S. Pat. No. 4,297,713 discloses a laser recording apparatus including a laser for emitting a laser beam and an optical unit for collimating the laser beam. In this laser recording apparatus, the distortion of a beam spot which arises from the beam-emitting angles determined with reference to both the vertical and horizontal directions is corrected by using a pair of cylindrical lenses arranged in close to the laser. Specifically, one of the cylindrical lenses is slid along the optical axis such that the cross section of the laser beam has a predetermined size and a predetermined shape.
In most of the presently-available laser diodes, however, the wavelength of a laser beam, the angle at which the laser beam is emitted, the efficiency at which the laser beam is emitted, and other parameters are adversely affected due to a temperature increase in the section where the laser diode is arranged. Thus, even when the optical unit shown in the U.S. Pat. No. 4,297,713 is used, it is likely that the cross section of the laser beam will not have a predetermined size or a predetermined shape.